This invention relates to the coating of members of graphite with a thin layer of an electrically highly conductive material from the vapor phase in general and more particularly to such a method using ion plating.
Graphite members to be coated are generally long carbon filaments or fibers, which may be combined in bundles. In a method, known from U.S. Pat. No. 3,550,247, for coating carbon filaments, the latter are first exposed to an aqueous oxidant and subsequently placed in a reaction solution which contains the material to be deposited, i.e., a metal. In a reduction reaction, the metal is then deposited on the pre-treated surfaces of the carbon filaments. Such fibers provided with a metal coating are required for assembling fiber contact brushes which are used for transmitting current between a stationary and a rotating part of an electric machine, for instance, a unipolar machine. With the known method, however, the danger exists that the individual fibers can stick together during the coating process and thus, only a non-uniform coating is obtained.
This danger, which exists particularly with very thin individual fibers, can also not be precluded completely by the method known from the British Pat. No. 1,309,252, although, in that method, the fiber bundle is conducted, prior to the coating process proper, through a chamber which is flared out in the direction of motion of the bundle and is exposed in the process to several jets of liquid arranged in series. Since the number of liquid jets, which are directed at right angles to the direction in which the bundle is led, steadily increases in the direction of motion of the bundle, the fibers are spread apart accordingly. This method, however, is relatively complicated and assumes that the fibers in the fiber bundle are not twisted.
With these methods of electroless deposition or electro deposition, only a very limited number of metals can furthermore be applied to the fibers. In addition, difficulties come up with this method in achieving satisfactory adhesion and high conductivity.
Furthermore, materials can also be deposited on graphite members by cathode sputtering ("Vakuumtechnik", 1975, pages 1 to 11). With such a method, a gas discharge process in a plasma is brought about between two electrodes. In the electric field required therefor, the gas ions are accelerated in a direction toward a cathode. When they arrive at the cathode surface with an energy of up to several keV, they release, on the one hand, secondary electrons which are necessary to maintain the gas discharge process, and, on the other hand, they knock out cathode material by means of collision processes. The usually electrically neutral particles of cathode material so produced diffuse through the gas and hit the member to be coated, which constitutes the anode, with average energies of several eV. These particles, however, can generally be precipitated only in a very thin layer, as the attainable condensation rate is relatively low as compared to pure vapor deposition and is, for instance, 100 times lower than with vapor deposition.
Particularly strict requirements apply to graphite containing members such as, for instance, fiber bundles which are to be used as brushes in electric machines. These requirements are, in particular, that electrically highly conductive materials, e.g., copper or silver, with nearly the conductivity of the solid material, must be applied in a relatively thin layer. In addition, good adhesion between the different materials is to be achieved. This is difficult, however, with metals such as copper, silver or gold which are not carbide forming. In addition, it should be possible to avoid corrosion phenomena of the applied layer materials. Optionally, the possibility of corrosion protection must therefore be provided. Furthermore, the friction along the individual graphite members should be small also if the graphite members are densely packed within a bundle, as the individual graphite member in such a bundle must be considered as an elastically suspended contact point which should follow the surface profile of the rotating machine part as exactly as possible.